Method for depositing an adhesive PVC layer on an electrode and electrode obtained according to said method

ABSTRACT

The invention relates to a method for depositing an adhesive PVC copolymer layer on a substrate, characterized in that it comprises the following steps: formation of a mixture of copolymer precursors in an organic solvent, whereby said mixture comprises PVC and 0.1-2 wt. % organotrialkoxysilane of formula H(HN—R 1 ) x —R 2 —Si—(OR 3 ) 3 , wherein R 1  and R 2  are alkyl groups or intermediate aromatic groups, R 3  is an aryl group, the three substituents R 3  cannot be the same and x is 0-2; a layer of said mixture is deposited on the substrate; the mixture is dried in order to evaporate the solvent; the aggregate thus obtained is heated to a temperature of 70-170° C. for a duration that respectively ranges from 3 hours and 5 minutes.

This application is a national state application under 35 U.S.C. 371 ofPCT/CH00/00205 which was filed on Apr. 10, 2000.

The present invention concerns a method for depositing an adhesive layerof a copolymer of polyvinyl chloride (PVC), notably on a hydrophilicmaterial, and even more specifically, on a material liable to react withan alkoxysilane. The invention also concerns an ion selectivemeasurement electrode, incorporating an adhesive layer produced by thismethod.

The present invention refers more specifically to a method fordepositing an adhesive PVC copolymer layer on an ion selectivemeasurement electrode, of a type incorporating a substrate, aninstrument designed to measure the activity of an ion in a solution anda membrane covering said instrument and formed of a PVC-base materialincorporating an ionophoretic agent selected as a function of the ion tobe measured.

PVC is a polymer having numerous applications. Unfortunately, bonding itto certain substrates poses a problem. This is the case, in particular,for ceramic, silicon and plastic substrates, which are hydrophilic,whereas PVC is hydrophobic.

U.S. Pat. No. 5,637,642 proposes a solution to this problem using acopolymer consisting of PVC and substitute alkyl trialkoxysilane,prepared by nucleophilic substitution at room temperature. The copolymeris placed in solution in an aprotic solvent, deposited on the substrateand then dried, again at room temperature. Such a copolymer is, it istrue, interesting, but the synthesis process proposed in this patent isexceptionally laborious and demands very strict production conditions,requiring the use of inert gas, the exclusion of water, and work in thedark.

The main purpose of the present invention is to propose a simple,effective method for bonding an adhesive PVC layer for less cost on asubstrate having a hydrophilic surface.

The method according to the invention is characterized in that itcomprises the following steps:

-   -   Formation, in an organic solvent, of a mixture of precursors of        the copolymer incorporating PVC and 0.1% to 2% by weight of an        organotrialkoxysilane corresponding to the formula:        H(HN—R₁)_(x)HN—R₂—Si—(OR₃)₃,        where R₁ is an intermediate alkyl or aromatic group, R₂ is also        an intermediate alkyl or aromatic group, R₃ is an alkyl group,        the three R₃ substituents may be not the same, and x is in the        range between 0 and 2.    -   Deposition of a layer of this mixture on the substrate.    -   Drying of the mixture to evaporate the solvent, and    -   Heating of the aggregate thus obtained to a temperature in the        70° C. to 170° C. range, for a time ranging between 3 hours and        5 minutes respectively.

Surprisingly, and despite the small thicknesses involved, it wasobserved that by performing polymerization of the copolymer directly onthe substrate and at a relatively high temperature, a perfectly adhesivelayer is obtained for a lower cost.

It is true that methods for depositing a PVC layer on a substrate arealready known. Such methods are, for example, described in documents DE14 94 534, GB 1 113 635, EP 0 045 396 and FR 2 438 076. In thesedocuments, a similar method is used to apply a PVC layer on a metalsubstrate, generally steel. The layers applied are thick, so as toprotect the metal surface.

It is preferable to select organotrialkoxysilane from 3-amino propyltriethoxysilane, 3(2-aminoethyl amino) propyl trimethoxysilane,trimethoxysilyl propyl diethylene triamine or their mixture. As avariant, organotrialkoxysilane can be formed of a mixture of3(2-aminoethyl amino) propyl triethoxysilane and phenyl triethoxysilane.

The tests performed showed that it was very beneficial to have atemperature close to 120° C. and applied for approximately two hours.

The quality of the PVC layer depends, in particular, on its flexibility.This characteristic is obtained thanks to the fact that the mixture ofcopolymer precursors includes, in addition, a plasticizer, in aproportion by weight of up to two parts plasticizer for one part PVC.Preferably, this proportion is one part PVC for one to two partsplasticizer.

The plasticizer can be chosen from dibutyl phthalate, bis(2-ethylhexyl)phthalate, dibutyl sebacate, bis(2-ethylhexyl) sebacate,tris(2-ethylhexyl) trimellitate, tris(2-ethylhexyl) phosphate, dioctylphenyl phosphonate, 2-nitrophenyl octyl ether, bis(1-butyl pentyl)decane-1,10-diyl diglutarate, tetraundecylbenzophenone-3,3′,4,4′-tetracarboxylate and their derivatives.

The choice of solvent depends on the type of PVC used. With a PVC ofhigh molecular weight, the solvent should preferably be selected frommethyl cyclohexanone, dimethyl formamide, nitrobenzene, isophorone,mesityl oxide, tetrahydrofurane and cyclohexanone. When a PVC of lowermolecular weight is used, the solvent should preferably be selected fromdipropyl ketone, methylamylketone, methyl isobutyl ketone,acetonylacetone, methyl ethyl ketone, dioxane, methylene chloride andtheir derivatives.

It is important that the mixture should be suitably viscous to be ableto be deposited on the substrate. Good results were obtained with onepart copolymer precursors for one to thirty parts of solvent.Preferably, the mixture contains one part copolymer precursors for tento twenty parts solvent.

PVC is used in particular as an ion selective membrane designed to equipISE type electrodes (Ion Selective Electrodes). Such electrodes can beused, for example, to measure the activity of ammonium, calcium,chloride, fluoroborate, nitrate, perchlorate and potassium ions, and tomeasure the hardness of water.

The membrane is generally formed of a mixture of high-molecular-weightPVC and a plasticizer, in a ratio of approximately 1 to 2, to which isadded an ionophoretic agent. The latter, which performs selectiveconduction of ions, is formed either of an ion exchanger or a neutraltransporter, in a proportion not exceeding 1% by weight. The mixturecan, moreover, be supplemented by adding lipophilic salts, selectedaccording to the electrode, the electrolyte and the application.

The use of PVC for manufacturing such membranes has proved especiallyeffective with large-sized electrodes, offering optimum measuringconditions.

The use of a so-called planar technology permits a considerablereduction in the size of the electrode, which comprises a polymer,ceramic or doped single-crystal silicon substrate, supporting ameasuring instrument produced by thin or thick layer deposition. Thismeasuring instrument can be formed of either a layer of metal or anion-sensitive field effect transistor (ISFET). It is preferable for theelectrode to be encapsulated in epoxy, with the exception of themeasuring instrument which is covered by an ion selective membrane.

This reduction in size permits a considerable reduction in the volume ofsolution to be analyzed. However, the bonding of a membrane, especiallya PVC membrane, poses a problem.

Another aim of the present invention is to produce an ion selectiveelectrode provided with a PVC membrane.

With an initial production method, such an electrode comprises asubstrate, an instrument designed to measure the activity of an ion in asolution, and a membrane covering the measuring instrument and formed ofa PVC-base material incorporating an ionophoretic agent selected as afunction of the ion to be measured. According to the invention, thiselectrode also incorporates a layer of a PVC and organotrialkoxysilanecopolymer produced by the method defined above, inserted between thesubstrate and the membrane, to ensure adhesion of the membrane.

Tests have shown that the membrane could be bonded directly onto themeasuring instrument. With a second production method, the electrodecomprises a substrate, an instrument designed to measure the activity ofan ion in a solution, and a membrane covering the measuring instrumentand formed of a PVC-base material incorporating an ionophoretic agentselected as a function of the ion to be measured. According to theinvention, this material is a PVC and organotrialkoxysilane copolymerproduced by the method outlined above.

The measuring instrument can be formed of either an ISFET or a metalliclayer.

Other characteristics and advantages of the invention will appear fromthe following description, making reference to the appended drawing, inwhich FIGS. 1 and 2 represent cross-section views of electrodesincorporating, as measuring instrument, a metallic layer and an ISFETrespectively.

FIG. 1 shows an electrode incorporating a substrate 10, preferably ofalumina, which carries a measuring instrument 12 formed, conventionally,of a layer of silver and silver chloride. The whole is covered with anadhesive layer 14, itself coated with a membrane 16 the characteristicsof which will be specified further on. An epoxy encapsulation, 18,protects the electrode, leaving merely a window opposite measuringinstrument 12.

The substrate 10 could also be made of another material than alumina,e.g. glass, silicon or a plastic material preferably consisting of apolymer reacting with the amine groups, especially polyamides and epoxyresins.

FIG. 2 shows another type of electrode in which the measuring instrumentis formed of an ISFET incorporating, in known manner, a substrate ofsingle-crystal silicon 20, a source 22, a drain 24 and a gate 26. Thelatter is coated with a membrane 28, self-adhesive, as will be seenlater. The ISFET can be mounted on a PC board and coated with epoxy,with the exception of the measurement surface. The surface of gate 26,which constitutes the sensible part of the measuring instrument, isgenerally formed of an oxide or nitride, e.g. a silicon dioxide, analuminium or tantalum oxide, or again a silicon nitride.

Note that an intermediate adhesive layer 14 (FIG. 1) or a self-adhesivemembrane 28 (FIG. 2) can be used indifferently with an ISFET or with ametallic electrode possibly incorporating interdigital structures suchas those used for conductometry measurements.

The construction of measuring instruments, of both the thin-layer andthick-layer metallic electrode type, and the ISFET type, is perfectlywell known in state-of-the-art practice and will therefore not bedescribed in greater detail.

The electrodes shown in the drawing are designed to determine theactivity of an ion in a solution, by measuring the potential differencebetween the electrode and a reference electrode, when they are plungedin the solution.

As was explained above, the materials forming membranes 16 and 28 playan essential role in the quality of measurement, the best results havingbeen obtained with PVC.

The major problem posed by the use of this material lies in how to bondmembranes 16 and 28.

In accordance with the invention, in the case of an electrode as perFIG. 1, membrane 16 is bonded on substrate 10 via layer 14, which is amixture formed of a PVC and organotrialkoxysilane copolymer with orwithout an added plasticizer.

More precisely, to form adhesive layer 14, one begins by producing, inan organic solvent, a mixture of precursors of the copolymerincorporating PVC and an organotrialkoxysilane. A layer of this mixtureis deposited on substrate 10 which is then placed for a few hours in alocation in which the atmosphere is normally moist, at room temperature.The solvent evaporates and atmospheric humidity causes (at leastpartial) hydrolysis of the alkoxysilane groups, so that the silane isbonded to the surface of the substrate. All this is then placed in aheat chamber, at a temperature in the range between 70° C. and 170° C.,for a time which is all the longer as the temperature is lower. In theseconditions, the PVC reacts with the organotrialkoxysilane to form acopolymer. As will be specified further on, the best results wereobtained with a treatment for two hours at 120° C.

The organotrialkoxysilane should be able to react with both thesubstrate and the copolymer so as to ensure bonding between one and theother. These two conditions are met when using an organotrialkoxysilanecorresponding to the formulaH(HN—R₁)_(x)HN—R₂—Si—(OR₃)₃,where R₁ is an intermediate alkyl or aromatic group, e.g. an ethylgroup, and R₂ is also an intermediate alkyl or aromatic group, e.g. apropyl, phenyl, phenoxypropyl or methyl phenyl. The three R₃substituents, which may or may not be different from one another, areformed by any alkyl group, e.g. the methyl, ethyl and propyl groups.Finally, x is in the range between 0 and 2. Note that the quantity oforganotrialkoxysilane corresponds to a proportion ranging between 0.1%and 2% of the total weight of the copolymer, preferably between 0.5% and1%.

A list of plasticizers which can be used for the production of ionselective PVC membranes can be found in the “Fluka selectophorecatalogue” of the firm Fluka Chemie AG, Buchs, Switzerland. Of theseplasticizers, preferably use dibutyl phthalate, bis(2-ethylhexyl)phthalate, dibutyl sebacate, bis(2-ethylhexyl) sebacate,tris(2-ethylhexyl) trimellitate, tris(2-ethylhexyl) phosphate, dioctylphenyl phosphonate, 2-nitrophenyl octyl ether, bis(1-butyl pentyl)decane-1,10-diyl diglutarate, tetraundecylbenzophenone-3,3′,4,4′-tetracarboxylate and their derivatives.

Typically, the proportion of plasticizer in the copolymer is 65%. It canbe considerably lower, and even null. In the latter case, the layerobtained is very stiff.

The solvents used to dissolve the PVC and plasticizers are thosegenerally used with PVC of high molecular weight. They include methylcyclohexanone, dimethyl formamide, nitrobenzene, isophorone, mesityloxide, tetrahydrofurane and cyclohexanone.

When the bonding layer is produced with PVC of lower molecular weight,preferably use a solvent chosen from dipropyl ketone, methylamylketone,methyl isobutyl ketone, acetonylacetone, methyl ethyl ketone, dioxane,methylene chloride and their mixtures.

The choice of solvent shall be made basically so that, when itevaporates, no water condensation forms on its surface. The quantity ofsolvent defines the mixture's viscosity, the optimum value of whichdepends on the method of deposition.

To produce ion selective membrane 16, use a polymer in a solutionpreferably formed of approximately ⅓ PVC and ⅔ plasticizer, supplementedby an ionophoretic agent, chosen according to the ions to be selected.The exact quantity of ionophoretic agent depends on the choice ofmaterials and application.

It generally represents approximately 1% by weight of the materialforming the membrane, i.e. without taking into account the solvent. Thecatalogue by the firm Fluka mentioned above gives all useful informationconcerning the choice of agent and the necessary quantity. The solventis selected from those mentioned above. This mixture is finally spreadon bonding layer 14.

In accordance with the invention, in the case of an electrode as perFIG. 2, ion selective membrane 28 is produced, directly on ISFET gate26, in the same way as layer 14 in FIG. 1, with the sole difference thatthe ionophoretic agent, in that case incorporated in membrane 16, is inthis case incorporated in the mixture during its preparation. It wasobserved, surprisingly, that such a membrane shows no major defect.Note, in particular, that ionophoretic agents withstand without anyproblem heat treatment at up to 170° C. without their characteristicsbeing altered. Moreover, no sensitivity to pH, which could have been dueto the introduction of an amino group, has been observed.

Various tests have been performed to test the method according to theinvention.

An initial series of tests was performed to define the roles played bythe choice of organotrialkoxysilane, the quantity used and the heattreatment temperature.

The basic solution was formed of 100 mg of PVC, 200 mg of dioctylsebacate (DOS) and 3 mg of organotrialkoxysilane dissolved in 3 ml ofcyclohexanone. Several types of organotrialkoxysilanes were tested,namely 3-aminopropyl triethoxysilane, 3(2-aminoethyl amino) propyltrimethoxysilane, trimethoxysilyl propyl diethylene triamine, and amixture of 3(2-aminoethyl amino) propyl trimethoxysilane and phenyltriethoxysilane, in a proportion of 1:1 by weight.

The solutions thus obtained were spread on glass plates for microscopesto form a layer whose thickness, in the range between 50 and 100 μm, wassufficient for testing the adhesion characteristics in good conditions.The assembly was dried at room temperature during one night in anormally moist atmosphere, then heated in a furnace as indicated. Theglass plates were then placed in water for at least two days.

The adhesion-related characteristics were assessed as follows:

− easy to remove +/− slight improvement + marked improvement ++ hard totear off, but the layer comes off in one piece when unstuck; +++ hard totear off, the layer is torn ++++ impossible to tear off.

The test results are summarized in the following table.

Quantity 70° C., 120° C., 170° C., Type of aminosilane [% by weight] 3 h2 h 5 min Without aminosilane − −/+ −/+ 3-aminopropyl 0.17 −/+ ++ +triethoxysilane 0.33 −/+ +++ ++ 0.66 −/+ ++++ ++ 1.33 −/+ ++++ +++3(2-aminoethyl amino) 0.17 + ++ + propyl 0.33 ++ ++++ ++trimethoxysilane 0.66 ++ ++++ ++ 1.33 ++ ++++ +++ Trimethoxysilyl 0.17 +++ + propyl diethylene 0.33 + +++ ++ triamine 0.66 + ++++ ++ 1.33 + +++++++ Mixture of silanes 0.09 −/+ ++ + 0.17 −/+ +++ + 0.33 + ++++ ++0.66 + ++++ ++ 1.33 ++ ++++ +++

It is obvious that, the higher the heat treatment temperature, theshorter the treatment must be. For example, deterioration of thecopolymer has been observed when the treatment exceeded five minutes at170° C., especially with a high concentration of organotrialkoxysilane.

At a temperature of 120° C. and for the highest concentration oforganotrialkoxysilane, the layer took on a yellow-brownish colour.

One observes, on examining the table, that the addition of anorganotrialkoxysilane considerably increases the PVC's adhesion toglass, especially with a concentration ranging between 0.5% and 1% andheat treatment for two hours at 120° C.

It is interesting to note that tests have been performed with treatmentfor three hours at 80° C. The adhesion obtained before submersion inwater was likewise excellent, but water droplets formed at the interfacebetween the PVC and glass, and the number of these droplets increasedsharply during the submersion.

Another test was performed using the base solution in the above example,with 1% by weight of 3(2-aminoethyl amino) propyl trimethoxysilane(relative to the total weight of PVC and plasticizer). An ISFET wascoated with a fine layer (5 to 10 μm) of this mixture, then placed in afurnace at 120° C. for two hours. The layer thus obtained was coveredwith an ion selective layer approximately 50 μm thick, produced from amixture formed of 100 mg of PVC, 200 mg of dioctyl sebacate (DOS) and 3mg of an ionophoretic agent, all this being dissolved in 3 ml ofcyclohexanone.

Various types of ionophoretic agents were used, namely valinomycin mixedwith 0.7 mg of potassium tetrakis(4-chlorophenyl) borate, nonactin andtetradodecyl ammonium nitrate. After evaporation of the solvent in theopen air for one night, the systems thus obtained were placed in anelectrolyte for several hours. After this, their ion selectivemeasurement properties were tested. The quality of the measurements wasin line with expectations.

The adhesion of the layers was then checked after submersion in theelectrolyte for a few days. All the parts covered with a copolymer layershowed excellent adhesion.

A third test was performed by producing a self-adhesive ion selectivelayer. This layer was prepared by mixing 100 mg of PVC, 200 mg ofdioctyl sebacate (DOS), 0.5 mg of 3-aminopropyl triethoxysilane and 3 mgof an ionophoretic agent in 1 ml of tetrahydrofurane. The sameionophoretic agents as those mentioned above were used.

The solution thus obtained was spread on the surface of the gate ofseveral ISFETs which had been first mounted on PC boards andencapsulated with epoxy. After evaporation of the solvent (one night inthe ambient air), the ISFETs were placed in a furnace at 170° C. forfive minutes, then submerged for at least two days in an appropriateelectrolyte. Here again, very good adhesion was observed, andmeasurement results fully meeting expectations.

Thus, surprisingly, the electrodes exposed to temperatures of up to 170°C., despite the risk of decomposition of the electroactive components,gave results in every way similar to those for electrodes obtained in aknown manner, with, moreover, excellent adhesion. In addition, noinfluence on pH was able to be detected in the regions tested, namely pH4-9 for K⁺ and NO₃ ⁻ ions and pH 4-7 for electrodes sensitive to NH₄ ⁻ions.

1. A method for depositing an adhesive PVC copolymer layer on anelectrode for ion selective measurement, the electrode comprising (a) asubstrate, (b) a measuring element for measuring the activity of an ionin a solution, and (c) a membrane covering said measuring element andformed of a PVC-based material incorporating an ionophoretic agentselected as a function of the ion to be measured, characterized in thatthe method comprises the steps of: forming, in an organic solvent, amixture of precursors of the copolymer incorporating PVC and 0.1% to 2%by weight of an organotrialkoxysilane corresponding to the formulaH(HN—R₁)_(x)HN—R₂—Si—(OR₃)₃, where: R₁ is an intermediate alkyl oraromatic group; R₂ is an intermediate alkyl or aromatic group; R₃ is analkyl group, with the three R₃ substituents possibly being not the same,and x is in the range from about 0 to 2, depositing a layer of themixture on said substrate, drying the mixture to evaporate the solvent,and heating the aggregate thus obtained to a temperature in a range fromabout 120° C. to about 170° C., for a time in a range from about 5minutes to about 3 hours.
 2. A method according to claim 1,characterized in that said organotrialkoxysilane is selected from3-amino propyl triethoxysilane, 3(2-aminoethyl amino) propyltrimethoxysilane, trimethoxysilyl propyl diethylene triamine andmixtures thereof.
 3. A method according to claim 1, characterized inthat said organotrialkoxysilane is formed of a mixture of 3(2-aminoethylamino) propyl trimethoxysilane and phenyl triethoxysilane.
 4. A methodaccording to claim 1, characterized in that said temperature is about120° C. and is applied for approximately two hours.
 5. An electrode forion selective measurement, comprising (a) a substrate, (b) a measuringelement for measuring the activity of an ion in a solution, and (c) amembrane covering said measuring element and formed of a PVC-basematerial incorporating an ionophoretic agent selected as a function ofthe ion to be measured, characterized in that it further comprises alayer of a PVC and organotrialkoxysilane copolymer inserted between thesubstrate and the membrane to ensure the latter's adhesion, and producedby the method according to claim
 4. 6. An electrode according to claim5, characterized in that said measuring element is formed of an ISFET.7. An electrode according to claim 5, characterized in that saidmeasuring element is formed of a metallic layer.
 8. An electrode for ionselective measurement, comprising (a) a substrate, (b) a measuringelement for measuring the activity of an ion in a solution and (c) amembrane formed of a PVC-base material incorporation an ionophoreticagent selected as a function of the ion to be measured, characterized inthat said material is a PVC and organotrialkoxysilane copolymer producedby the method according to claim
 4. 9. An electrode according to claim8, characterized in that said measuring element is formed of an ISFET.10. An electrode according to claim 8, characterized in that saidmeasuring element is formed of a metallic layer.
 11. A method accordingto claim 1, characterized in that said mixture includes, in addition, aplasticizer, in a proportion by weight of up to two parts plasticizerfor one part PVC.
 12. A method according to claim 11, characterized inthat said proportion is one part by weight of PVC for one to two partsplasticizer.
 13. A method according to claim 11, characterized in thatsaid plasticizer is selected from dibutyl phthalate, bis (2-ethylhexyl)phthalate, dibutyl sebacate, bis(2-ethylhexyl) sebacate,tris(2-ethylhexyl) trimellitate, tris(2-ethylhexyl) phosphate, dioctylphenyl phosphonate, 2-nitrophenyl octyl ether, bis (1-butyl pentyl)decane-1, 10-diyl diglutarate, tetraundecylbenzophenone-3,3′,4,4′-tetracarboxylate and their derivatives.
 14. Amethod according to claim 1, characterized in that said solvent isselected from methyl cyclohexanone, dimethyl formamide, nitrobenzene,isophorone, mesityl oxide, tetrahydrofurane and cyclohexanone, and thatthe PVC is of a high molecular weight type.
 15. A method according toclaim 14, characterized in that said mixture contains one part by weightof precursors of said copolymer for one to thirty parts of solvent. 16.A method according to claim 15, characterized in that said mixturecontains one part by weight of precursors of said copolymer for ten totwenty parts of solvent.
 17. A method according to claim 1,characterized in that said solvent is selected from dipropyl ketone,methylamylketone, methyl isobutyl ketone, acetonylacetone, methyl ethylketone, dioxane, methylene chloride and their mixtures, and in that thePVC is of a low molecular weight type.